As we scale to deep sub-micron (DSM) technology, transistor threshold frequencies increase, enabling the design of complementary metal oxide semiconductor (CMOS) circuits for radio frequency (RF) and mm-wave applications up to 67 GHz. However, such high-frequency CMOS transistors have very limited gain, resulting in poor output power. A successful transition into DSM CMOS applications therefore requires high-Q, low-power components operating at high frequencies.
Another challenge facing DSM circuits is the increasing density of devices, projected to reach 1011 devices/cm2. At such densities, clock distribution and the power consumption associated with it necessitate implementation of low-power local clocks with the potential for global synchronization.
There are currently electromechanical resonators and oscillators in the market taking advantage of the high quality factors of acoustic resonators to try to solve the above problems in CMOS design. The highest performance products available are at SiTime® (www.sitime.com), but have a limited frequency range of 1-125 MHz. The SiTime products are off-chip with dimensions ˜1 mm2. They do not incorporate transistor action into the body of the resonator.
In 1967, Nathanson et al. demonstrated the Resonant Gate Transistor (in IEEE Trans. Electron Devices, Vol. 14, pages 117-133) driving resonance in a conductive gold cantilever with an air-gap capacitive electrode. The Resonant Gate Transistor (RGT) cantilever functions as the gate of an airgap transistor, with output drain current modulated by the cantilever resonant motion. Resonant Gate transistors were demonstrated with frequencies up to 100 kHz.
In 2003, Leland Chang introduced the concept of a Resonant Body Transistor (RBT) in his Ph.D. thesis in the Electrical Engineering and Computer Science department at University of California, Berkeley. (L. Chang, Nanoscale Thin-Body CMOS Devices,” Chapter 8, PhD. Dissertation in Electrical Engineering and Computer Science, University of California, Berkeley, Spring 2003.) As illustrated in FIG. 1, Chang proposed an air-gap flexural mode RBT 30 composed of two clamped-clamped beams 32 coupled together at the two anchor points 34, 36 (double-ended tuning fork configuration). The geometry resembles an air-gap dual-gate FinFET with two fins 38, 40. One fin 38 is biased into accumulation and the other 40 into strong inversion. The device operates as follows:
(1) The top fin 38 is biased in accumulation (−VGate). No current flows across this fin 38, but a capacitive force (Fcap,ac˜VGateVinCOX1/g) from the excitation Vin drives resonant motion.
(2) Mechanical vibrations couple the top fin to the bottom fin through the anchors 34, 36 on either end of the beams 32. The bottom fin 40 resonates out of phase with the top fin 38.
(3) The bottom fin 40 is biased in strong inversion (+VGate). As the bottom fin 40 moves, COX2 varies, modulating the drain current IDrain.
Unfortunately, there are several obstacles to scaling Chang's air-gap flexural mode RBT 30 for frequencies greater than 10 GHz, such as difficulties obtaining smaller air gaps and difficulties preventing stiction.
Therefore, it would be desirable to have a reliable Resonant Body Transistor that could be scaled for use at very high frequencies, well above the 10 GHz range, that was also practical to fabricate in order to enable the design of deep sub-micron circuits for RF applications.